Only create destination folders if they do not already exist

gruntjs/grunt-contrib-sass#66

Only create destination folders if they do not already exist. The change prevents a previously created CSS file being overwritten by an empty one and this change detected by a live reloader reloading a blank CSS file.
Merge pull request #98 from andypattenden/master

Only create destination folders if they do not already exist
v0.7.3